The present invention is directed to a process for the production of ceramic green parts.
The following five forming methods are normally used to produce ceramic green parts:
1) pressing
2) isopressing
3) extrusion
4) slip casting and
5) injection molding.
The selection of the forming method is based on the shape, function, and number of pieces of the required part.
Free flowing, easily pressed starting powders are required for pressing and isopressing. For extrusion of parts with axial symmetry, powder mixtures with organic or inorganic plastifiers are used. As in polymer processing, injection molding is needed for complex shaped green parts. This process uses a mixture of a ceramic powder and a thermoplast which is put into a mold, and heated after forming to burn off the thermoplastic additives.
In slip casting, the ceramic powder and water are mixed with deflocculants and binders to give a free flowing slip, which is then poured into porous molds. The extraction of water by the mold (i.e. molds of gypsum or porous plastics) leads to the formation of green parts, which are dried after demolding and are subsequently sintered. This well known process is used to produce hollow green parts. Although useful for a variety of applications, it has several disadvantages:
1) the hardening of the green body depends upon the extraction of water; Therefore migration of soluble slip components may occur, resulting in an unequal particle distribution in the shaped green part,
2) the forming process is slow, and
3) density gradients are observed in the green body.
In addition, the final products possess low green strength and the green body workability is limited. Careful and time consuming drying of the porous mold is necessary before reuse. In addition, the casting of complex shapes is difficult and undercuts are hard to achieve.
Several attempts have been made in recent years to improve ceramic processing by transforming homogenous powder suspensions into solid like green bodies. This can be achieved by either consolidating the dispersion medium or by flocculating or coagulating the suspension particles.
Consolidating the dispersion medium is achieved in gel-casting when monomers in the suspension are polymerized. Polymerization can be induced by UV radiation, applying heat or by catalysts. In freezing casting the dispersion medium is consolidated by freezing and removed after demolding by sublimation.
Flocculation and coagulation of a highly dense packed particle system uses the control of interparticle forces in order to accomplish a liquid-solid transition. Flocculation and coagulation may by induced either by applying heat, or by changing the ionic strength of the suspension. A steric stabilized poser particle suspension has been used which becomes destabilized upon heating to form a rigid green body. One may first coagulate a suspension by adding salt, then increase the solids loading of this destabilized suspension to form a coagulated, plastic behaving, clay like mass which, after destabilization, fills in the mold. One may also work along the same line to produce a coagulated, clay like mass which, after destabilization can be vibrated into a mold to form the green body which is then demolded. The process is time consuming and produces unreliable ceramic parts. In this process the powder particle network which is formed during coagulation is disturbed during the shaping process by deforming the clay like mass into the mold.
The present invention has as its object an improved slip casting process which eliminates these disadvantages.
Pursuant to the invention this object is solved and there is provided a process for the production of ceramic green parts from a castable, aqueous slip wherein an active substance is added to the slip which changes the surface of the slip particles leading to solidification of the slip.
The invention provides a technically simple, quick, and inexpensive alternative to other known forming methods. In contrast to conventional slip casting, improvements in forming complex shaped green parts are possible, while the required microstructure, surface properties and mechanical properties are maintained. For example, this new slip casting process very largely suppresses migration and heterogeneous distribution of soluble components as the hardening step is not dominated by water withdrawal. In addition, the development of density gradients are avoided as the forming process is faster than in normal slip casting.